Hydrogen transport mechanism through dense metal

Fig. 10.6 Illustration of the accepted hydrogen transport mechanism through dense metal membranes...

Hydrogen can permeate selectively dense metal layers, and, accordingly, this phenomenon is exploited when metal membranes are applied for separating ultrapure hydrogen from gas mixtures [24-27]. The hydrogen mass transfer through dense metal membranes includes several transport mechanisms, which are discussed in the following sections. 

As explained in Chapter 5, the transport mechanism in dense crystalline materials is generally made up of incessant displacements of mobile atoms because of the so-called vacancy or interstitial mechanisms. In this sense, the solution-diffusion mechanism is the most commonly used physical model to describe gas transport through dense membranes. The solution-diffusion separation mechanism is based on both solubility and mobility of one species in an effective solid barrier [23-25], This mechanism can be described as follows first, a gas molecule is adsorbed, and in some cases dissociated, on the surface of one side of the membrane, it then dissolves in the membrane material, and thereafter diffuses through the membrane. Finally, in some cases it is associated and desorbs, and in other cases, it only desorbs on the other side of the membrane. For example, for hydrogen transport through a dense metal such as Pd, the H2 molecule has to split up after adsorption, and, thereafter, recombine after diffusing through the membrane on the other side (see Section 5.6.1). 

Transport Mechanism of Hydrogen Through Dense Metal Membranes and Governing Permeation Equations... 

Researchers have reported the influence of various chemical species on the performance of palladium-based membranes. Observed reductions in hydrogen flux through dense metal membranes are typically attributed to the blocking of the adsorption of molecular hydrogen, which is a necessary step in the atomic transport mechanism [13, 80,112,114,120-123],... 

Hydrogen can permeate selectively dense metal membranes, behaviour that permits the separation of hydrogen from gas mixtures. The mass transfer mechanism consists of several steps dissociation of hydrogen molecules into atoms, interaction of hydrogen atoms with the metal surface and their adsorption, diffusion of hydrogen into the metal lattice, and desorption of hydrogen atoms from the other metal surface and their recombination into molecules.The overall transport process through the metal wall is called permeation and is ruled by the expression ... 

Hydrogen transport through a dense metal membrane is usually envisioned as following an atomic transport mechanism. The atomic transport process, assuming transport from the high hydrogen pressure surface to the low pressure surface. 

Defect free dense metal membranes are infinitely selective to hydrogen due to the atomic transport mechanism that can be described through derivation of Pick s Law (defining the transient mass transfer of hydrogen in the direction of decreasing concentration). 

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